1. Field of the Invention
The present invention relates to an a-Si TFT liquid crystal device, a liquid crystal display panel and a method of manufacturing them.
2. Description of Related Art
A conventional method for manufacturing an a-Si TFT liquid crystal display panel will now be described with regard to one liquid crystal device portion. As shown in FIGS. 15(a) and 15(b), a gate electrode 4 is first formed into a predetermined shape on a glass substrate 2 by a general technique. Then, as shown in FIG. 16, a gate insulating film 6, a channel layer 8 and a SiNx film 10 serving as an etching stopper layer are successively stacked on the entire surface of the glass substrate 2. As shown in FIGS. 17(a), 17(b) and 17(c), after coating a resist on the SiNx film 10, the glass substrate 2 is exposed to light from its back surface side by using the gate electrode 4 in the predetermined shape as a light shielding mask, and is subsequently subjected to stepper exposure from its front surface side by using a reticle, and then the resist is developed. Thereafter, the SiNx film 10 other than a portion working as an etching stopper (channel protecting film) 14 is etched with diluted hydrofluoric acid, and then the resist is removed.
It is noted that the etching stopper 14 can be basically formed through a single exposing step in which the glass substrate 2 is subjected to the stepper exposure from its front surface side by using a reticle. In this conventional manufacturing method, however, the etching stopper 14 is formed through a two-stage exposing step: a stage of exposing the substrate 2 to light from its back surface side and a stage of exposing the substrate 2 to light from its front surface side. This is because, when the etching stopper 14 is formed through a single-stage exposing step in which the glass substrate 2 is exposed to light from its front surface side, the alignment with the gate electrode 4 tends to be shifted and cannot be stabilized. In contrast, if the gate electrode 4 is effectively used in the two-stage exposing step, the etching stopper 14 can be disposed at the center of the gate electrode 4 in a self-alignment manner. This results in providing a source electrode 26 and a drain electrode 28 symmetrically about the gate electrode 4, and also in reducing overlap areas between the gate electrode 4 and the drain electrode 28 and between the gate electrode 4 and the source electrode 26. Thus, the two-stage exposure can improve a transistor characteristic.
However, in manufacturing a liquid crystal display panel, there are a large number of complicated manufacturing steps, and in addition, each steps requires time-consuming work. Therefore, reduction in the number of processes not only improves the productivity but also reduces the manufacturing cost of a liquid crystal display panel in which the process cost accounts for a large proportion.
After being developed, the resist used for forming the etching stopper 14 has a size of approximately 20×10 μm per pixel, and such rectangular resists are arranged side by side over the entire surface of an array substrate. Since the area of each resist is thus small, its adhesion to the underlying nitride film (i.e., the SiNx film) is low, and hence, the resist is apt to be easily peeled off. When the resist is peeled off, the etching stopper 14 cannot be properly formed, which leads to a transistor failure.
When the etching stopper layer 10 is etched with diluted hydrofluoric acid, the layer 10 is generally over-etched so as not to leave an insufficiently etched portion. However, excessive over-etching makes the side surface of the etching stopper 14 be inclined inward at the foot thereof, resulting in the formation of a “concave” 15, which is hidden in a top view as shown in FIG. 17(c). When films and/or foreign matter to be deposited in subsequent steps are attached to the concave 15, they cannot be removed through cleaning and etching. As a result, as shown in FIG. 18, a leakage current flows between the source electrode 26 and the drain electrode 28 formed on the etching stopper 14, which leads to a leakage failure of the transistor.